Combustion chambers form part of gas turbines, which are used in many fields to drive generators or machines. In such applications the energy content of a fuel is used to generate a rotational movement of a turbine shaft. For this purpose the fuel is combusted by burners in the combustion chambers connected downstream of them, with compressed air being supplied by an air compressor. Combustion of the fuel produces a high-temperature working medium which is subject to high pressure. This working medium is directed into a turbine unit connected downstream from the combustion chambers, where it expands in a manner that provides work output.
In this arrangement a separate combustion chamber can be assigned to each burner, whereby the working medium flowing out of the combustion chambers can be combined before or in the turbine unit. Alternatively the combustion chamber can however also be designed as what is known as an annular combustion chamber structure, in which a majority, in particular all, of the burners open out into a common, typically annular, combustion chamber. The turbine unit adjacent to the combustion chamber in the direction of flow of the working medium typically comprises a turbine shaft which is connected to a plurality of rotatable blades which form series of blades in an overlapping ring shape. The turbine unit also comprises a plurality of fixed vanes which are also attached in an overlapping ring shape to the inner housing of the turbine thereby forming series of vanes. The blades serve here to drive the turbine shaft by transmitting the pulse from the working medium flowing through the turbine unit, while the vanes serve to direct the flow of the working medium between two consecutive series of blades or blade rings viewed in the direction of flow of the working medium in each instance.
As the rotational movement of the turbine shaft is generally used to drive the air compressor connected upstream of the combustion chamber, this is extended beyond the turbine unit so that the turbine shaft is surrounded in a toroidal manner by the annular combustion space in the area of the annular combustion chamber connected upstream from the turbine. The combustion space is bounded in this case by an annular outer wall on the one hand and an annular inner wall located within it on the other hand. For this purpose the inner wall of the combustion chamber generally comprises two or more individual parts which are screwed together on their side facing the turbine shaft.
In the design of gas turbines of this kind a particularly high level of efficiency is one of the design aims in addition to the achievable performance. Here, an increase in efficiency can basically be achieved for thermodynamic reasons through an increase in the exit temperature at which the working medium flows out of the combustion chamber and into the turbine unit. For this reason temperatures of around 1200° C. to 1500° C. are aimed at for gas turbines of this kind and also attained.
With the working medium reaching such high temperatures, however, the components and parts exposed to this medium are subject to high thermal stresses. In order nonetheless to ensure a comparatively long useful life for the affected components, in particular the combustion chamber, while maintaining high reliability, it is usually necessary to construct them using particularly heat-resistant materials and to provide a means of cooling them. In order to prevent thermal deformations of the material which limit the useful life of the components, efforts are usually made to achieve as uniform a cooling of the components as possible.
For this purpose the combustion chamber wall can be lined on its inside with heat shield elements which can be provided with particularly heat-resistant protective layers and which are cooled through the actual combustion chamber wall. Toward that end, a cooling method also known as “impingement cooling” can be employed. With impingement cooling a cooling medium, generally cooling air, is supplied to the heat shield elements through a plurality of holes drilled in the combustion chamber wall so that the cooling medium impinges essentially vertically onto its external surface facing the combustion chamber wall. The cooling medium heated up by the cooling process is subsequently discharged from the inner space that the combustion chamber wall forms with the heat shield elements.
The manufacture of a cooling system of this type can be very expensive, however, since very many holes with a comparatively small cross-section are needed in the combustion chamber wall in order to achieve as uniform a cooling of the heat shields as possible, which can be very time- and cost-intensive. In particular the requirements to be met by the tools needed to produce the holes are very high, since the cooling air holes are relatively long compared to their cross-section because the structure of the combustion chamber wall must have a sufficiently great strength for stability reasons. Furthermore, with a large number of cooling air holes which in total add up to a large surface area, there is a possibility of friction and turbulence occurring in the supply of the cooling medium. This leads to an increased cooling medium pressure loss in the cooling medium circuit, which has a disadvantageous effect on the efficiency of the combustion chamber.
Moreover the design of the annular combustion chamber described above has a number of further disadvantages with regard to necessary maintenance work. With these maintenance and repair activities, which are generally performed at regular intervals, it is necessary to repair or replace parts of the combustion chamber such as, for example, the heat shield elements or the cooling system used as well as in particular also components of the downstream turbine unit because of the high thermal and mechanical loads to which they are exposed. A disadvantage in the design of the combustion chamber is that the turbine shaft is not accessible from the combustion chamber when maintenance work is carried out. Consequently, in order to perform maintenance work on the turbine shaft in the area of the annular combustion chamber or to carry out repairs to the first vanes and blades immediately adjacent to the combustion chamber, it is usually necessary to remove all the contiguous vanes and blades of the turbine unit. Only after all vanes and blades of the turbine have been disassembled is it possible to remove the inner wall of the combustion chamber by way of the screw connection facing the turbine shaft and so gain access to the turbine shaft. The assembly work is therefore very labor- and time-intensive. As a result of the comparatively long operational outage of the gas turbine, downtime costs are incurred in addition to the assembly costs for the gas turbine, leading to comparatively very high total costs for maintenance and repair work to the gas turbine.